Field of the Invention
The present invention relates to drug-eluting medical devices; more particularly, this invention relates to processes for making a drug-eluting, polymer-based medical device.
Background of the Invention
Strict pharmacological and good mechanical integrity of a drug eluting medical device are required to assure a controlled drug release. Significant technical challenges exist when developing an effective and versatile coating for a drug eluting medical device, such as a stent.
The art recognizes a variety of factors that might play a significant role in a stent coating's ability to retain its integrity when subjected to external loadings, such as crimping and balloon expansion. According to the art, many of these factors are not fully understood, nor their significance fully appreciated. To the inventor's knowledge, the art has only been able to characterize the mechanisms of action as pertains to coating integrity at a theoretical level. A discovery of a relationship between the material, process used to coat, deformation of the stent in relation to that of the coating, etc. that might reduce the inherent uncertainty in this field, however, has yet to be articulated. As will be appreciated, the fracture mechanics for a coating is a highly non-linear problem that cannot be reduced down to a few variables.
Nevertheless, the art has tried to develop a methodology capable of reducing the fracture problem down to a few unknowns—e.g., choosing a temperature range, a crimping protocol, and a polymer having the desired elastic properties as a function of the stent and clinical/therapeutic objectives. For example, attempts have been made to predict inter-laminar shear, crack propagation and related structural-mechanical behavior of a coating when subjected to an external load using a finite element-based analytic approach. To the inventors' knowledge, however, none of these efforts have proven successful, at least to the extent that their results might enable those in the medical device field to predict with any useful degree of certainty whether a particular drug-eluting stent will suffer from cracks, peeling, etc. in its coating absent device-specific testing.
In light of the foregoing, there is a need for a method that improves the integrity or resilience of a drug-polymer coating carried by a polymer-based medical device, such as a polymer stent, when the medical device is subjected to external loads. It is important to avoid cracking and/or peeling of a drug-polymer coating. Potentially, such coating damage during stent radial expansion/contraction can cause severe thrombogenic response after stent implantation and compromise control of the drug dosage and/or rate of release of the drug-eluting stent.